A permanent magnet VFD motor is the most expensive single component on an OPPAIR compressor, and the least forgiving to take apart. The rotor carries a strong permanent field, the stator windings sit only millimetres away, and bearing preload depends on how the end shields seat. Done in the right order the motor goes back together as new; rushed, you score a stator or lose a rotor to a steel bench.
This page covers the complete motor service cycle in one place: preparation, disassembly, inspection, shaft seal replacement, reassembly with staged torque, and the checks that must pass before the drive is energised. Both OPPAIR workshop videos are embedded below — the first is the full strip-and-assemble walk-through, the second is the seal replacement on the IP23 frame.
Video 1: OPPAIR PM VFD motor disassembly and assembly, 2:39. If the player is blocked on your network, open it directly on YouTube.
What this guide covers
- Preparation and lockout — isolating the drive, discharging the DC bus, marking cables and the mechanical clock position.
- Tools and workspace — what has to be on the bench before the first bolt comes out.
- Disassembly — fan and cowl, terminal box, end shields, rotor withdrawal.
- Inspection — windings, magnets, bearings, shaft journal, encoder and cooling path.
- Shaft seal replacement — removing the old seal, installing the new one, protecting the lip.
- Reassembly and torque — seating the shields, staged tightening, coupling alignment.
- Checks before restart — insulation resistance, free rotation, no-load and loaded run.
Before you touch a bolt
Almost every PM motor failure that arrives at our service desk is a procedure failure, not a wear failure. These points are the ones that cost money.
- Isolate and prove dead. Open the isolator, lock it, tag it, and confirm zero volts at the motor terminals with a meter — not from the drive display.
- Let the drive discharge. A VFD holds a lethal DC bus charge after the supply is removed. Wait the period printed on the drive label before the terminal cover comes off.
- Record everything before you disconnect. Photograph U, V, W, earth, encoder and thermistor cores, and mark the end shields against the frame so they go back in the same clock position. A PM motor reverses or faults instantly if the sequence changes.
- Control the rotor. The rotor is permanently magnetised. Keep it away from steel benches, hand tools, watches and phones, and never set it down where swarf can jump onto the magnets.
- Plan the lift. Frame sizes used on 30 kW to 55 kW machines are not a one-person lift; use a hoist rated for the mass on the nameplate.
- Release the machine first. Air pressure bled off, oil drained, coupling disconnected, pipework capped.
- Order the seal by motor model and serial number. The seal is matched to shaft diameter, bore and the oil it runs in — not to what looks close on the shelf.
Bench layout and tools
Set the bench up before the motor arrives. Working out of a toolbox mid-job is how seals get cut and magnets pick up filings.
| Item | Purpose on this job |
|---|---|
| Puller with bearing separator | Rotor withdrawal and bearing removal without loading the windings |
| Calibrated torque wrench | End shields, terminal studs, coupling bolts — one wrench covering the range in the manual |
| Seal driver or square-faced sleeve | Drives the seal in square; never tap a seal in with a hammer |
| Socket and hex set, feeler gauges, dial indicator | Fasteners, coupling alignment, bearing clearance checks |
| Insulation tester (megger) | Proves the windings are dry and clean before re-energising |
| Lint-free cloths, cleaning solvent, assembly grease | Bore and face cleaning, lip lubrication, corrosion protection |
| Non-magnetic tray and labelling tape | Fasteners in removal order, cores tagged, small parts contained |
Keep compressed air away from the open stator. Blowing into a winding cavity drives grit into the insulation and between the magnets and the bore, which is exactly the debris you cannot see afterwards.
Disassembly, in order
- Photograph the machine in its installed state — cable routing, coupling gap, fan orientation. This is the reference you will check against at the end.
- Disconnect and label power cores, earth, encoder and thermistor leads. Cap the terminal box so nothing drops in.
- Drain oil and bleed air, then split the coupling. Check the coupling element while it is out; a worn element shows up later as vibration you will blame on the motor.
- Remove the fan cowl and fan. Note the fan direction arrow before it comes off the shaft.
- Mark the end shields against the frame, then release the shield bolts in a cross pattern, in stages, so the housing does not spring.
- Withdraw the rotor axially, straight and slow, with the puller — never by levering against the windings. Support the rotor the moment it clears the stator bore: the magnetic pull between rotor and laminations is strong enough to damage both if the rotor is allowed to snap sideways.
- Bag and tag the bearings, shims, circlips and seal as they come off, and record which side each bearing came from.
What to inspect while it is open
| Component | What to look for | Action |
|---|---|---|
| Stator windings | Oil tracking, discolouration, odour, loose lacing, debris between coils | Clean and dry; replace the motor if insulation resistance stays low |
| Rotor magnets | Chips, lifting coating, embedded ferrous dust, heat discolouration | Clean with a non-metallic scraper; replace the rotor if a magnet is chipped |
| Bearings | Rough rotation, axial play, blue or brown staining, seal leakage | Replace as a pair — a noisy bearing has already contaminated its neighbour |
| Shaft journal | A polished groove at the seal contact line, scoring, keyway damage | Repair sleeve or new shaft; a new seal alone will not hold on a grooved journal |
| Encoder / resolver | Cracked housing, oil ingress, damaged connector pins | Replace; do not attempt to re-pot a contaminated encoder |
| Cooling path | Blocked fins, deformed cowl, fan erosion | Clean and straighten; replace a fan with missing blades |
Shaft seal replacement: the IP23 job
Seal failure is the most common reason a PM motor comes off the machine in the first place. Oil reaches the windings through the seal, insulation resistance falls, and the drive trips long before anyone sees oil on the floor. The second video shows the seal job on an IP23 PM inverter motor, stripped and rebuilt with a single puller tool.
Video 2: OPPAIR IP23 PM inverter motor disassembly, main oil seal replacement and reassembly, 4:07. Watch it on YouTube if the player is blocked.
| Chapter | Stage | What to watch for |
|---|---|---|
| 0:00 | Workshop intro | Layout of the motor before disassembly — photograph this state |
| 0:05 | Safe disassembly | Puller placement, rotor withdrawal, winding protection |
| 1:02 | Removing the old seal | How the seal is levered out without scoring the bore |
| 2:08 | Installing the new seal | Square insertion, lip orientation, spring position |
| 3:01 | Reassembly and QC | Bearing and end-shield fit, manual rotation check |
Removing the old seal
Seal removal is where most repeat failures are created. The objective is to take the seal out without touching the bore or the shaft.
- Clean the area around the seal first so nothing falls into the bearing cavity.
- Lever the seal out with a flat, blunt tool against the seal's metal case — never against the aluminium housing bore. Work around the circumference in small increments rather than prying from one point.
- If the seal is tight, a small self-tapping screw wound into the case gives the puller a grip point.
- Wipe the bore and inspect it for scoring, burrs or sealant residue. A raised burr will cut the new lip during installation.
- Inspect the shaft at the lip contact line: a polished groove deeper than a fingernail catch means a new seal alone will not hold, and the shaft or a repair sleeve is needed.
Installing the new seal
- Clean the bore with a solvent that leaves no residue, then let it dry completely.
- Confirm the direction before the seal touches the housing: the spring side faces the oil, the lettering and flat face outward. Fitting a seal backwards produces an immediate leak.
- Lubricate the lip with clean assembly grease — the same oil the motor runs in, not a general-purpose grease that attacks the elastomer.
- Protect the lip as it passes over the shaft: a wrap of tape or a thin plastic sleeve over the keyway and thread prevents a sharp edge from cutting the rubber.
- Press the seal in square with a driver of even diameter, applying force to the outer case only, until it sits at the same depth as the original. Crooked or too deep blocks the oil return path.
- Wipe away excess sealant immediately; cured sealant inside the housing becomes debris in the bearing.
Reassembly and torque control
Assembly is where the quality of the job is decided. Clean faces, correct seating and controlled torque matter more than the brand of bearing you fit.
- Clean every mating face on the shields and frame. Grit trapped between them changes bearing preload and shows up as an axial rumble within weeks.
- Fit bearings with the right method — press on the inner ring only, or heat if the manual allows it. Never drive a bearing home through the outer ring on a shaft fit.
- Refit the rotor straight, without letting it drag or snap against the stator. Keep the insertion axis the same as the withdrawal axis.
- Seat the shields in the marked positions, then tighten in a cross pattern in at least two stages: snug first, then final torque with the calibrated wrench.
- Follow the manual for torque values, and check whether the figures are specified dry or lightly lubricated. The same bolt needs a different setting depending on thread condition, and mixed assumptions around one bolt circle is how housings crack.
- Reconnect the coupling and check alignment with a feeler gauge or dial indicator before the machine is bolted down for good. Misalignment is the most common cause of a repeat bearing failure.
- Reconnect the cores exactly as photographed, and re-check every terminal for tightness — a loose encoder pin produces faults that look like drive failures.
Checks before the motor is energised
- Insulation resistance to earth with a megger. A low reading means moisture or oil has reached the windings: clean and dry, do not simply re-energise.
- Hand rotation. The shaft must turn freely with no rub, notch or end-float step.
- Fastener audit. Every bolt you touched, checked once more against the manual values.
- No-load run for 30 minutes. Watch the shaft end for the first weep, and log bearing temperature and vibration against the readings taken before the overhaul.
- Loaded run for one shift. A dry shaft end and a stable temperature trend after two hours of loaded running is the acceptance criterion for the seal job.
Faults that look like motor faults but are not
| Symptom after overhaul | Likely cause | Check first |
|---|---|---|
| Drive trips on overcurrent at start | Phase sequence wrong or encoder offset not set | Recorded core order, drive parameter set |
| Runs but reverses | Two cores swapped | U/V/W labelling from the photos |
| High vibration at running speed | Coupling misalignment or damaged coupling element | Dial indicator on the coupling |
| Bearing temperature climbing | Over-greased, wrong grease, or shield not seated square | Grease quantity, shield gap, preload |
| Oil weep returns within days | Seal fitted backwards, lip cut on the keyway, or worn journal | Lip orientation, insertion depth, journal wear groove |
| Weep returns within a month | Root cause is alignment, vibration, oil temperature or wrong oil | Coupling alignment, oil spec, running temperature |
| Burn smell after first start | Lip fitted dry — it burned on the first rotation | Assembly grease on lip and shaft contact line |
| Low insulation resistance | Oil or moisture in the windings, damaged lead | Megger per phase, visual inspection |
When to send it to us instead
Do the job in-house if you have the puller, a calibrated wrench and a clean bench. Stop and call service if any of the following is true:
- The shaft shows a groove or the bore is scored — the repair is no longer a seal swap.
- The stator is contaminated, or insulation resistance stays low after cleaning and drying.
- A magnet is chipped, or the rotor has taken an impact.
- The frame is beyond what you can lift and support safely.
Parts, motors and related guides
- Motors — PM VFD motors and replacement units by frame size
- All spare parts — bearings, seals, encoders and service kits
- PM VSD compressors with inverter — the machines this motor drives
- Air-end oil seal replacement — the companion procedure on the compressor side
- Replacing the main unit on a PM integrated compressor
Send us the motor model, serial number and a photo of the shaft end and we will confirm the correct seals, bearings and encoder for your frame through the contact page. OPPAIR supplies export builds from 30-day standard delivery, with up to three years of factory warranty on IP65 PM VSD core assemblies.